Research project: Analysis of debonding failure of externally–bonded FRP systems on concrete

Currently Active:

Yes

Premature debonding of Fibre Reinforced Polymer (FRP) plates hampers the efficient use of externally-bonded FRP plates for flexural strengthening of concrete beams. The uncertainty about the governing mechanisms of FRP debonding means that there is no reliable theory that can be applied by designers.

Since flaws are inevitable in the interface, what matters is whether an existing flaw can propagate causing debonding. We developed a fracture mechanics based model that represents energy balance requirements, rather than an unreliable analysis of the crack-tip stress field. The model is based on governing parameters that can be reliably determined. The model is able to analyse all modes of FRP debonding. Methods to determine the energy states in beams and the interface fracture energy to a reliable accuracy were also developed. The debonding model predicts results that match with experimental test results of FRP debonding on concrete beams.

Fig 3

Our model will be used to answer the question “Will this interface crack extend?” This presupposes that a preexisting crack must be present. It is not concerned with how that crack or flaw forms in the first place.

Fig. 3 – Debonding crack just (a) before (b) after small a extension

Results

Using the concept of, ‘the current state of a system will be at a position of minimum total potential energy’, the model determines that debonding will occur if the energy available for a potential small extension of an existing interface crack exceeds the energy needed to form the required new fracture surfaces – that is, if the energy release rate (energy release per unit extension of a crack of unit width) associated with the crack exceeds the interface fracture energy then the crack will propagate. It is sufficient to assume that flaws of the relevant size are likely to exist. The model can be used to determine the shortest crack that triggers failure at a given load, or the failure load of a beam with a crack of known length.

Fig 4

Fig. 4 – Failure loads for different plate curtailment locations of a given simply supported beam

Benefits to structural engineering

The model we developed provides an essential tool that will enable fracture mechanics to be used to determine the load at which FRP plates will debond from concrete beams. This will obviate the need for finite element analyses to be used in situations where there is an infinite stress concentration and where the exact details of the interface geometry and properties are unknowable.